Lecture the semantic_web_part_2

The Semantic
Web – Part 2
Semantic CMS Community

Lecturer
Organization
Modeling Complex Knowledge
Date of presentation Domains

Co-funded by the
1 Copyright IKS Consortium
European Union

Page:

Part I: Foundations

(1) Introduction of Content Foundations of Semantic
(2)
Management Web Technologies

Part II: Semantic Content Part III: Methodologies
Management

Knowledge Interaction Requirements Engineering
(3) (7)
and Presentation for Semantic CMS

(4) Knowledge Representation
and Reasoning
(8)
Designing
Semantic CMS

Semantifying
(5) Semantic Lifting (9) your CMS

Storing and Accessing Designing Interactive
(6) Semantic Data
(10) Ubiquitous IS

www.iks-project.eu Copyright IKS Consortium

Page: 3

Semantic Web Layer Cake

A language for querying
information specified in
RDF.

A model for describing
resources with properties A format for specifying structured
and property values. data in a machine-readable form

Unique identification
of resources
Semantic Web Layer Cake, Image source: http://www.w3.org/2007/03/layerCake.svg

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RDF
 RDF can be used to make statements about single
resources identified by URIs
 RDF is able to define relations among resources
 Literals can be assigned as values to properties of
resources
http://example.org/rel/Brand
http://example.org/myCar http://example.org/Jaguar

http://example.org/rel/Model
http://example.org/myCar XF


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RDF Classes
http://example.org/rel/Brand
http://example.org/myCar http://example.org/Jaguar

Car
Vehicle http://example.org/VW

How are these terms related Do these terms have
to each other? anything in common?


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RDF Properties
http://example.org/my http://example.org/rel/Brand http://example.org/University
Car Paderborn

Is “UniversityPaderborn” a valid
value for property “Brand”?

 We need to do statements about properties  What is the
type of the property?
 Definition of valid types for subjects and objects
 Definition of hierarchies of properties


Page: 7


Page: 8

RDF Schema
 “RDF'svocabulary description language, RDF
Schema, is a semantic extension of RDF. It provides
mechanisms for describing groups of related resources
and the relationships between these resources.”
 W3C Recommendation
 RDF Vocabulary Description Language 1.0: RDF Schema

 The RDFS namespace is:
“rdfs: http://www.w3.org/2000/01/rdf-schema#”
 But RDFS also uses elements from the rdf
namespace! http://www.w3.org/TR/rdf-schema/


Page: 9

RDF Schema
 RDFS does not provide a specific vocabulary for
describing properties such as “Brand”, but a language
to define vocabularies

 RDFS uses RDF to define RDF
vocabularies

 Due
to it's limited expressiveness,
RDFS is a language to define lightweight ontologies

http://www.w3.org/TR/rdf-schema/

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RDFS Classes
 “Resources may be divided into groups called classes.
The members of a class are known as instances of the
class. Classes are themselves resources.”

 Themembers (resources) of a class A are termed
instances of the class A

 Classes are resource, e.g.
“http://example.org/CarManufacturer” and can be
described by RDF properties


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RDFS Classes
 Thepredicate rdf:type assigns the object as type of the
subject

 Theassignment to a type is not exclusive
 A subject can be typed by several objects!

 Example:

ex:Jaguar rdf:type ex:CarManufacturer


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RDFS Classes
 A resource can be defined as a class explicitly
 The type rdfs:Class defines a resource as a class

 Example:

ex:CarManufacturer rdf:type rdfs:Class

 rdfs:Class is the “class of all classes”, i.e.:

rdfs:Class rdf:type rdfs:Class


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Hierarchies of Classes
 “Theproperty rdfs:subClassOf is an instance of
rdf:Property that is used to state that all the instances
of one class are instances of another.”
 The subject and object of the predicate
“rdfs:subClassOf” must be instances of rdfs:Class
 The property rdfs:subClassOf is transitive
 Example:

ex:CarManufacturer rdfs:subClassOf
ex:Company

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RDFS Constructs - Predefined Classes
All things described by RDF are called resources, and
rdfs:Resource
are instances of the class rdfs:Resource.
This is the class of resources that are classes.
rdfs:Class
rdfs:Class is an instance of rdfs:Class.
The class rdfs:Literal is the class of literal values such
rdfs:Literal as strings and integers. rdfs:Literal is an instance of
rdfs:Class. rdfs:Literal is a subclass of rdfs:Resource.
rdfs:Datatype is the class of datatypes. rdfs:Datatype is
rdfs:Datatype both an instance of and a subclass of rdfs:Class. Each
instance of rdfs:Datatype is a subclass of rdfs:Literal.
The class rdf:XMLLiteral is the class of XML literal
rdf:XMLLiteral values. rdf:XMLLiteral is an instance of rdfs:Datatype
and a subclass of rdfs:Literal.
Typos?
rdf:Property is the class of RDF properties. rdf:Property
rdf:Property
is an instance of rdfs:Class.
http://www.w3.org/TR/rdf-schema/

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RDFS Properties
 “TheRDF Concepts and Abstract Syntax specification
describes the concept of an RDF property as a relation
between subject resources and object resources”

 A resource can be defined as property explicitly

ex:Brand rdf:type rdf:Property


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Hierarchies of Properties
 “The property rdfs:subPropertyOf is an instance of
rdf:Property that is used to state that all resources
related by one property are also related by another.”
 The subject and object of the predicate
“rdfs:subPropertyOf ” must be instances of
rdf:Property
 The property rdfs:subPropertyOf is transitive
 Example:

ex:friendOf rdfs:subPropertyOf ex:knows

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Property Restrictions

http://example.org/my http://example.org/rel/Brand http://example.org/University
Car Paderborn

Is “UniversityPaderborn” a valid
value for property “Brand”?

 Weneed to express, that only subjects and objects of
specific types should be used for the predicate “Brand”


Page: 18

Domain
 “rdfs:domain is an instance of rdf:Property that is
used to state that any resource that has a given
property is an instance of one or more classes.”
 States that each subject with the predicate (property) is
an instance of the defined class
 Ifmore than one domain is defined, the subjects are
instances of all classes!
 Example:

ex:Brand rdfs:domain ex:Vehicle

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Range
 “rdfs:range is an instance of rdf:Property that is used
to state that the values of a property are instances of
one or more classes.”
 States that each object of the predicate (property) is an
instance of the defined class
 If more than one range is defined, the objects are
instances of all classes!
 Example:

ex:Brand rdfs:range ex:Company


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RDFS Constructs – Predefined
Properties (I)
rdfs:range is an instance of rdf:Property that is used to
rdfs:range state that the values of a property are instances of
one or more classes.
rdfs:domain is an instance of rdf:Property that is used
rdfs:domain to state that any resource that has a given property is
an instance of one or more classes.
rdf:type is an instance of rdf:Property that is used to
rdf:type
state that a resource is an instance of a class.
The property rdfs:subClassOf is an instance of
rdfs:subClassOf rdf:Property that is used to state that all the instances
of one class are instances of another.
The property rdfs:subPropertyOf is an instance of
rdfs:subPropertyOf rdf:Property that is used to state that all resources
related by one property are also related by another.
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Copyright IKS Consortium

Page: 21

RDFS Constructs – Predefined
Properties (II)
rdfs:label is an instance of rdf:Property that may be
rdfs:label used to provide a human-readable version of a
resource's name.
rdfs:comment is an instance of rdf:Property that may
rdfs:comment be used to provide a human-readable description of a
resource.

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Copyright IKS Consortium

Page: 22

RDFS - Example
class
class
ex:Company
ex:Vehicle rdf:range
rdfs:domain

rdfs:subClassOf
ex:Brand rdfs:subClassOf
class property
ex:Car class
ex:CarManufacturer

RDFS
rdf:type
Terminological knowledge
RDF
rdf:type Assertional knowledge

ex:Brand
ex:myCar ex:Jaguar


Page: 23

Back to the Cake ...

A language for querying
A language for describing a
RDF.
lightweight ontology.

of resources

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Why is RDFS not enough?
 RDFS cannot express negations

 Defined property restrictions are global

 Missing cardinalities for properties

 Relations between (sub-)classes (e.g. disjunction)


Page: 25

OWL – Web Ontology
Language
 “The OWL Web Ontology Language is designed for use
by applications that need to process the content of
information instead of just presenting information to
humans.”
 OWL has been developed as a vocabulary extension of
RDF
 Explicitly
represents the meaning of terms in
vocabularies and the relationships between those
terms. (Ontology)

http://www.w3.org/TR/2004/REC-owl-features-20040210/

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OWL – The Story
 2004 - OWL W3C Recommendation
 2009 - OWL 2 W3C Recommendation

OWL = Web Ontology Language
 Why not WOL?
 Obvious pronunciation which is easy on the ear http://piqs.de
 Opens up great opportunities for logos
 Owls are associated with wisdom
 It has an interesting back story
http://lists.w3.org/Archives/Public/www-webont-wg/2001Dec/0169.html

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Dialects of OWL
 OWL Lite
 Provides classification hierarchy and simple constraint
features
 OWL DL (description logic) - Includes OWL Lite
 Provides the maximum expressiveness without losing
computational completeness and decidability of reasoning
systems.
 OWL Full - Includes OWL DL
 Is meant for users who want maximum expressiveness
and the syntactic freedom of RDF with no computational
guarantees.
http://www.w3.org/TR/2004/REC-owl-features-20040210/

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How does OWL look like?
RDF Document
 Basic structure of an OWL Document Namespace
Definitions
<rdf:RDF xmlns:owl ="http://www.w3.org/2002/07/owl#"
xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
Default Namespace
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:xsd ="http://www.w3.org/2001/XMLSchema#">
xmlns="http://example.org/">
Ontology Header
<owl:Ontology rdf:about=“”>
<rdfs:comment>An example OWL ontology</rdfs:comment>
<owl:priorVersion rdf:resource="http://example.org/20120101/cars"/>
<owl:imports rdf:resource="http://example.org/20110707/manufacturer"/>
<rdfs:label>Car Ontology</rdfs:label>
</owl:Ontology>
Importing another
...
ontology
</rdf:RDF>


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Simple OWL Classes
 Comparable to RDFS classes
 Every individual in OWL is a member of the class
owl:Thing
 Each user-defined class is implicitly a subclass of
owl:Thing
 OWL also defines the empty class owl:Nothing
 Definition of domain-specific root classes:

<owl:Class rdf:ID=“Vehicle”/>
<owl:Class rdf:ID=“CarManufacturer”/>

Define class

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Hierarchies of Classes
 Thedefinition of basic taxonomies for classes can be
done by rdfs:subClassOf.

 rdfs:subClassOf is transitive.
“Car” is a subclass
of “Vehicle”
<owl:Class rdf:ID=“Car">
<rdfs:subClassOf rdf:resource="Vehicle"/>
</owl:Class>


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Disjoint Classes
 “The disjointness of a set of classes can be expressed
using the owl:disjointWith constructor. It guarantees
that an individual that is a member of one class cannot
simultaneously be an instance of a specified other
class.“ The classes “Car”
and “Vehicle” are
disjoint.
<owl:Class rdf:about=“Car">
<owl:disjointWith rdf:resource=“Cat”/>
</owl>


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Infer about Disjoint Classes
The class “Car” is a
<owl:Class rdf:ID=“Car"> subclass of “Vehicle”
<rdfs:subClassOf rdf:resource="Vehicle"/>
</owl:Class>

The class “Cat” is a
<owl:Class rdf:ID=“Cat"> subclass of “Animal”
<rdfs:subClassOf rdf:resource=“Animal"/>
</owl:Class>
The classes
<owl:Class rdf:about=“Vehicle"> “Vehicle” and
<owl:disjointWith rdf:resource=“Animal”/> “Animal” are disjoint
</owl>

It can be inferred, that the classes “Car” and “Cat“
are disjoint too!

Page: 33

Individuals
 In addition to the definition of classes, we also need to
describe the members of classes (individuals)
Definition of
<owl:Thing rdf:ID=“Jaguar"/> individual “Jaguar”

<owl:Thing rdf:about=“Jaguar">
<rdf:type rdf:resource=“CarManufacturer"/>
</owl:Thing>
Ties the individual
to the class

 Abbreviated representation
< CarManufacturer rdf:ID="Jaguar"/>


Page: 34

Simple Properties
 “This world of classes and individuals would be pretty
uninteresting if we could only define taxonomies.
Properties let us assert general facts about the
members of classes and specific facts about
individuals.“
 Property are binary relations. In OWL, two different
types of properties are distinguished:
 object properties, relations between instances of two
classes.
 datatype properties, relations between instances of
classes and RDF literals and XML Schema datatypes
http://www.w3.org/TR/owl-guide/

Page: 35

Object Properties
 Objectproperties relate instances of classes by
rdfs:domain and rdfs:range.
Properties are
<owl:ObjectProperty rdf:about=“Brand”/> defined like classes

Domain and range
<owl:ObjectProperty rdf:ID=“Brand”>
for object property
<rdfs:domain rdf:resource=“Vehicle”/> “Brand”
<rdfs:range rdf:resource=“CarManufacturer”/>

</owl:Thing>


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Datatype Properties
 Datatypeproperties relate class instances and data
values using rdfs:domain and rdfs:range.
Properties are
defined like classes
<owl:DatatypeProperty rdf:about=“ConstructionYear”/>

Domain and range for
<owl:ObjectProperty rdf:ID=“ConstructionYear”>
datatype property
<rdfs:domain rdf:resource=“Car”/> “ConstructionYear”
<rdfs:range rdf:resource=“&xsd;gYear”/>

</owl:Thing>

http://www.w3.org/TR/owl-guide/

Page: 37

Recommended XML Schema
datatypes
xsd:string xsd:normalizedString xsd:boolean
xsd:decimal xsd:float xsd:double
xsd:integer xsd:nonNegativeInteger xsd:positiveInteger
xsd:nonPositiveInteger xsd:negativeInteger xsd:byte
xsd:long xsd:int xsd:short
xsd:unsignedLong xsd:unsignedInt xsd:unsignedShort
xsd:hexBinary xsd:base64Binary xsd:unsignedByte
xsd:dateTime xsd:time xsd:date
xsd:gYear xsd:gMonthDay xsd:gDay
xsd:anyURI xsd:token xsd:language
xsd:NMTOKEN xsd:Name xsd:NCName
xsd:gYearMonth xsd:gMonth


Page: 38

Cardinality Constraints for
Properties
 owl:minCardinality defines the minimal cardinality for
the value of a property.
 owl:maxCardinality defines the maximal cardinality for
the value of a property.
<owl:Class rdf:ID=“Vehicle">
Anonymous class Restriction for
<rdfs:subClassOf> property “Wheels”
<owl:Restriction>
<owl:onProperty rdf:resource=“Wheels”/>
<owl:minCardinality rdf:datatype=“&xsd;nonNegativeInteger”>2
</owl:minCardinality>
</owl:Restriction>
</rdfs:subClassOf>
Minimal number of
</owl:Class> “Wheels” is “2”


Page: 39

Cardinality Constraints for
Properties
 “owl:cardinality
permits the specification of exactly the
number of elements in a relation.”

<owl:Class rdf:ID=“Motorcycle">
Restriction for
<rdfs:subClassOf> property “Wheels”
<owl:Restriction>
<owl:onProperty rdf:resource=“Wheels”/>
<owl:cardinality rdf:datatype=“&xsd;nonNegativeInteger”>2
</owl:cardinality>
</owl:Restriction>
</rdfs:subClassOf>
Exact number of
</owl:Class> “Wheels” is “2”


Page: 40

Property Restrictions
 “The owl:allValuesFrom restriction requires that for
every instance of the class that has instances of the
specified property, the values of the property are all
members of the class indicated by the
owl:allValuesFrom clause.” Restriction on property
 Local restriction for class definition “PreOwner“ for
instances from class
<owl:Class rdf:ID=“Car">
<rdfs:subClassOf>
“Car”
<owl:Restriction>
<owl:onProperty rdf:resource=“preOwner” />
<owl:allValuesFrom rdf:resource=“Person” />
</owl:Restriction>
<rdfs:subClassOf>
</owl:Class> Each value is member
of class “Person” Copyright IKS Consortium
www.iks-project.eu

Page: 41

Property Restrictions Restriction on property
<owl:Class rdf:ID=“Car"> “PreOwner“ for
instances from class
<rdfs:subClassOf> “Car”
<owl:Restriction>
<owl:onProperty rdf:resource=“preOwner” />
<owl:someValuesFrom rdf:resource=“Person” />
</owl:Restriction>
<rdfs:subClassOf> At least one value is
</owl:Class> member of class
“Person”

Relation Implication
allValuesFrom For all cars, if they have a previous owner, all
previous owners are persons.
someValuesFrom For all cars, they have at least one previous
owner that is a person.

Page: 42

Property Restrictions with
Constants
 “owl:hasValue allows us to specify classes based on
the existence of particular property values. Hence, an
individual will be a member of such a class whenever at
least one of its property values is equal to the
owl:hasValue resource.”
<owl:Class rdf:ID=“myCars"> Restriction for
<rdfs:subClassOf>
property “Owner”
<owl:Restriction>
<owl:onProperty rdf:resource=“Owner”/>
<owl:hasValue rdf:”JohnMiller”>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>
The class “myCar”
is defined by value
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assignment Copyright IKS Consortium

Page: 43

Intersection
 owl:intersectionOf defines a class as an intersection
of other classes.

<owl:Class rdf:ID=“CarsFromVW">

<owl:intersectionOf rdf:parseType=“Collection”>
<owl:Class rdf:about=”Cars”>
<owl:Class rdf:about=“VW”/> If something is a “Car”
</owl:intersectionOf> and a “VW” it is a
</owl:Class> ”CarFromVW”


Page: 44

Union
 “owl:unionOf describes a class that contains those
individuals that occur in at least one of the class
Gearbox is everything that
extensions of the class descriptions in the list”.
is “ManualTransmission”
or
<owl:Class rdf:ID=“Gearbox"> “AutomaticTransmission”.
<owl:unionOf rdf:parseType=“Collection”>
<owl:Class rdf:about=”ManualTransmission”>
<owl:Class rdf:about=“AutomaticTransmission”/>
</owl:intersectionOf>

</owl:Class>


Page: 45

Complement
 “The owl:complementOf construct selects all
individuals from the domain of discourse that do not
belong to a certain class.”

<owl:Class rdf:about=”ManualTransmission”>

<owl:Class rdf:about=“AutomaticTransmission”/>
</owl:complementOf ref:resource=“ManualTransmission” />
</owl:Class>

“AutomaticTransmission”
includes as members all
individuals that do not
belong to
“ManualTransmission”.

Page: 46

If you want to know more ...
 There is stuff about OWL out there, which is not part of
this course:
 Class axioms
 Global restriction for properties
 Logical characteristics of properties

 Ifyou are interested in learning more about OWL please
check:
 The W3C OWL Reference (http://www.w3.org/TR/owl-ref)
 The W3C OWL Guide (http://www.w3.org/TR/owl-guide)


Page: 47

OWL 2
 OWL 2 adds new functionality to OWL, while the overall
structure is very similar
 OWL 2 provides syntactic sugar
 e.g. disjoint union of classes
 Some OWL 2 feature provide new expressivity:
 keys, property chains
 richer datatypes, data ranges;
 qualified cardinality restrictions
 asymmetric, reflexive, disjoint properties
 enhanced annotation capabilities
http://www.w3.org/TR/owl2-primer/

Page: 48

Back to the Cake ...

Highly expressive ontology
language for modelling complex
A language for querying knowledge domains.
RDF. A language for describing a
lightweight ontology.

of resources

Page: 49

Lessons Learned
 Now you should know ...
 ... what is the main difference between RDF and ontology
languages like RDF Schema and OWL.
 ... how can RDF Schema be used to build simple
ontologies.
 ... what are the shortcomings of RDF Schema and why is
it not enough in some application cases.
 ... the web ontology language OWL.
 .. in which sense OWL is more expressive than RDF
Schema.


Page: 50

References and Additional
Material
 RDFVocabulary Description Language 1.0: RDF
Schema
 http://www.w3.org/TR/rdf-schema/
 OWL Web Ontology Language Guide
 http://www.w3.org/TR/owl-guide/
 OWL Web Ontology Language Reference
 http://www.w3.org/TR/owl-ref/
 OWL 2 Web Ontology Language Primer
 http://www.w3.org/TR/owl2-primer/


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